Intheheliumdilutiontechnique,aknownconcentration(C1)ofaninertgas(suchashelium)isaddedtoaboxofknownvolume(V1).Theboxisthenconnectedtoavolume(V2)thatisunknown(thelungvolumetobemeasured).Afteradequatetimefordistributionoftheinertgas,thenewconcentration(C2)oftheinertgasismeasured.Thechangeinconcentrationoftheinertgasisthenusedtodeterminethenewvolumeinwhichtheinertgashasbeendistributed(
Fig.21.4 ).Specifically,
Inbodyplethysmograph(bodybox),RobertBoyle’sgaslaw—thatpressuremultipliedbyvolumeisconstant(ataconstanttemperature)—isusedtomeasurelungvolumes.Thepatientsitsinanairtightbox(
Fig.21.5 )andbreathesthroughamouthpiecethatisconnectedtoaflowsensor(pneumotach).Thepatientthenmakesapantingrespiratoryeffortagainstaclosedmouthpiece.Duringtheexpiratoryphaseofthemaneuver,thegasinthelungbecomescompressed,lungvolumedecreases,andthepressureinsidetheboxfallsbecausethegasvolumeintheboxincreases.Oncethevolumeoftheboxandthechangeinpressureoftheboxatthemouthareknown,thechangeinvolume(ΔV)ofthelungcanbecalculated: whereP1andP2aremouthpressuresandVisFRC.FromthemeasurementofFRC,inspiratorycapacitycanberecordedasthevolumeofairinspiredaboveFRC,andexpiratoryreservevolumecanbedeterminedasthevolumeofgasexhaledfromFRC.Thesemeasurementscanthenbeusedtodeterminetheotherlungvolumes.
•Fig. 21.5 The Body Plethysmograph. Notethattheboxinwhichthepatientsitsisnotdepicted.
Determinants of Lung Volume
What determines the volume of air in the lung at TLC or at RV? The answer lies in the properties of the lung parenchyma and in the interaction between the lungs and the chest wall. The lungs and chest wall always move together as a unit in healthy individuals. The lung contains elastic fibers that (1) stretch when stress is applied, which results in an increase in lung volume, and (2) recoil passively when this stress is released, which results in a decrease in lung volume. The elastic recoil of the lung parenchyma is very high. In the absence of external forces (such as the force generated by the chest wall), the lungs become almost airless (10% of TLC). Similarly, chest wall volume can increase when the respiratory muscles are stretched and decrease when respiratory muscle length is shortened. In the theoretical absence of the lung parenchyma, the resting volume of the chest wall increases and is approximately 60% of TLC.
Lung volumes are determined by the balance between the lung’s elastic properties and the properties of the muscles of the chest wall. The maximum volume of air contained within the lung and the chest wall (i.e., TLC) is controlled by the muscles of inspiration (see ). With increasing lung volume, the chest wall muscles lengthen progressively. As these muscles lengthen, their ability to generate force decreases. TLC occurs when the inspiratory chest wall muscles are unable to generate the additional force needed to further distend the lung and chest wall. Similarly, the minimal volume of air in the –40–20 0 20 40 Pressure (cm H2O) (PL, P , or P ) •Fig. 21.6 Relaxation Pressure-Volume Curve of the Lung, Chest Wall, and Respiratory System. Thecurvefortherespiratorysystemisthesumoftheindividualcurves.Thecurveforthelungisthesameastheoneforthenormallungin